A conventional electro-absorption modulator is configured, for example, as illustrated in a schematic sectional view of FIG. 5, as a modulator having a modulator structure formed on an n-type InP substrate 100 and made from a material substantially lattice matching with the InP substrate.
In particular, as illustrated in FIG. 5, the conventional electro-absorption modulator is structured such that an n-type InP cladding layer 101, an n-side optical guide layer (SCH layer) 102, an undoped multiple quantum well (MQW) active layer 103, a p-side optical guide layer (SCH layer) 104, a p-type InP cladding layer 105 are stacked in order on the n-type InP substrate 100 to form a mesa structure and the mesa structure is buried in a high-resistance InP burying layer 106. Further, a p-side electrode 107 is formed on the upper face of the p-type InP cladding layer 105 and an n-side electrode 108 is formed on the back face of the n-type InP substrate 100. Further, a reverse bias electric field is applied to the MQW active layer 103 by the upper and lower electrodes 107 and 108 to vary the absorption coefficient for light propagating in the MQW active layer 103 thereby to modulate (intensity modulate) the intensity of the light.
Here, an InGaAsP-based material is conventionally used for the MQW active layer 103 and also for the SCH layers 102 and 104 which sandwich the MQW active layer 103 from below and above.
In this instance, the energy band diagram along a broken line A-A in FIG. 5 is such as illustrated in FIG. 6. In particular, as seen in FIG. 6, the energy band gap is the smallest in a well layer of the MQW active layer 103 and successively increases in order of a barrier layer of the MQW active layer 103, the n-side and p-side SCH layers 102 and 104, and the n-type and p-type InP cladding layers 101 and 105.
Particularly, in the n-side and p-side SCH layers 102 and 104, the energy band gap varies, as seen in FIG. 6, so as to gradually increase from the side on which the layers 102 and 104 contact with the MQW active layer 103 (barrier layer) toward the side on which the layers 102 and 104 contact with the n-type and p-type InP cladding layers 101 and 105, respectively. Such variation of the energy band gap as just described can be implemented by changing the composition of InGaAsP while a lattice matching condition with the InP substrate is satisfied.
By introducing such a graded layer whose composition continuously varies in this manner into the SCH layers (SCH graded layers), the energy level continuously varies on both of the p side and the n side such that the MQW active layer 103 whose energy band gap is small and the InP cladding layers 101 and 105 whose energy band gaps are large are smoothly connected to each other on the energy band diagram. In short, as a structure for connecting layers having energy band gaps different from each other, the structure is implemented which eliminates the discontinuity (band discontinuity, band offset) of the energy level between the conduction band and the valence band.
Incidentally, in recent years, it is studied to use an AlGaInAs-based material which is one of materials which lattice match with an InP substrate as a material for an MQW active layer similarly to an InGaAsP-based material.
This is because, different from an InGaAsP-based material, an AlGaInAs-based material can improve a device performance (for example, an extinction ratio, a temperature characteristic or the like) of an electro-absorption modulator arising from the fact that, where a layer of a composition having a large band gap and another layer of another composition having a small band gap are connected to each other, the band offset of the conduction band and the band offset of the valence band are different from each other.
Also it is studied to use, where an InGaAsP-based SCH graded layer and an AlGaInAs-based MQW active layer which are made from materials different from each other are connected to each other, in order to eliminate the band discontinuity appearing on the interface between the layers, an AlGaInAs-based material same as that of the MQW active layer for the p-side SCH graded layer.